Optical communication circuits

ABSTRACT

Various apparatuses, circuits, systems, and methods for optical communication are disclosed. In some implementations, an apparatus includes a package substrate and f first interposer mounted on the package substrate. The apparatus also includes a logic circuit and an optical interface circuit connected to the logic circuit via the first interposer. One of the optical interface circuit or the logic circuit is mounted on the first interposer. The optical interface circuit includes a driver circuit configured to receive electronic data signals from the logic circuit. The optical interface circuit also includes an optical transmitter circuit coupled to the driver circuit and configured to output optical data signals encoding the electronic data signals.

FIELD OF THE DISCLOSURE

The disclosure generally relates to high speed communication, and moreparticularly to optical communication.

BACKGROUND

Fiber optics are used in a number of applications for high speed datacommunication. Communication systems based on fiber optics transmit dataas modulated laser light through an optical fiber (e.g., glass orplastic). Fiber optic communication systems are advantageous for manyapplications as noise is not induced in the fiber by the presence ofelectromagnetic signals in the environment.

SUMMARY

Various apparatuses, circuits, systems, and methods for opticalcommunication are disclosed. An apparatus is disclosed that includes apackage substrate and a first interposer mounted on the packagesubstrate. The apparatus also includes a logic circuit and an opticalinterface circuit connected to the logic circuit via the first. One ofthe optical interface circuit or the logic circuit is mounted on thefirst interposer. The optical interface circuit includes a drivercircuit configured to receive electronic data signals from the logiccircuit. The optical interface circuit also includes an opticaltransmitter circuit coupled to the driver circuit and configured tooutput optical data signals encoding the electronic data signals.

A method is also disclosed for manufacturing an apparatus having anoptical communication circuit. A logic circuit is mounted on a firstinterposer. An optical interface circuit is formed on a secondinterposer by mounting an optical transmitter circuit on the secondinterposer, mounting a driver circuit on the second interposer, andconnecting the driver circuit via wiring on the second interposer. Thefirst interposer is mounted on a substrate having one or more wiringlayers. The second interposer is mounted on the substrate. The logiccircuit die and the optical interface circuit are connected via thefirst interposer, the one or more wiring layers, and the secondinterposer.

An apparatus having an optical serializer is also disclosed. The opticalserializer includes a plurality of optical modulators. Each of theoptical modulators is configured to receive a respective bit of aparallel multi-bit data bus in a first bit period. Each of the opticalmodulators is configured to output a respective optical pulserepresenting the value of the respective bit. The optical pulse has aduration less than the first bit period. For each of the opticalmodulators, the optical serializer includes an optical delay lineconfigured to delay the optical pulses output from the optical modulatorto produce a respective optical output signal. Each optical delay linedelays pulses by a respective length of time unique to the opticalmodulator connected thereto. The optical serializer also includes anoptical combiner configured to combine the respective optical outputsignals produced by the optical delay line into a single optical beam.

A method for optical serialization is also disclosed. For each bit ofparallel multi-bit data bus transmitted in a first bit period, arespective optical modulator is used to provide a respective opticalpulse. The optical pulse has a duration less than the first bit period.Each of the respective optical pulses is delayed by a respective lengthof time unique to the optical modulator to produce a respective opticaloutput signal. The respective optical output signals are combined into asingle optical beam.

Other features will be recognized from consideration of the DetailedDescription and Claims, which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and features of the disclosed methods, circuits, andsystems will become apparent upon review of the following detaileddescription and upon reference to the drawings in which:

FIG. 1 shows an IC package including a logic circuit and a serializercircuit mounted on a substrate via a first interposer and an opticalinterface circuit mounted on the substrate via a second interposer;

FIG. 2 shows an IC package including a logic circuit and a serializercircuit mounted directly on a substrate and an optical interface circuitmounted on the substrate via an interposer;

FIG. 3 shows an IC package including a logic circuit and a serializercircuit mounted on a substrate via an interposer and an opticalinterface circuit mounted directly on the substrate;

FIG. 4 shows an IC package including a logic circuit mounted on asubstrate via a first interposer and a serializer and optical interfacecircuits mounted on the substrate via a second interposer;

FIG. 5 shows an IC package including a logic circuit mounted directly ona substrate and a serializer and optical interface circuits mounted onthe substrate via an interposer;

FIG. 6 shows an IC package including a logic circuit mounted on asubstrate via an interposer and a serializer and optical interfacecircuits mounted directly on the substrate;

FIG. 7 shows an optical serializer, in accordance with one or moreimplementations;

FIG. 8 shows a configurable optical transmitter, in accordance with oneor more implementations; and

FIG. 9 shows a programmable IC that may be configured, in accordancewith one or more implementations.

DETAILED DESCRIPTION OF THE DRAWINGS

Various aspects of the present disclosure are believed to be applicableto a variety of different types of apparatuses, circuits, systems, andmethods involving optical communication.

Various integrated circuit (IC) packages are disclosed that include alogic circuit and an optical communication circuit. One or both of thelogic circuit and optical communication circuit are assembled onrespective interposers for later mounting and connection in an ICpackage. This allows the logic circuit and optical communication circuitassemblies to be separately manufactured and tested prior to finalassembly in an IC package. Accordingly, flawed logic circuits and/oroptical communication circuits can be identified before beingmanufactured into the final IC package. In this manner, the productionyield for manufacture of the IC package is increased.

The disclosed implementations are applicable IC packages includingvarious logic circuits in combination with an optical communicationcircuit. For example, in some applications, the logic circuit mayinclude a programmable IC. For ease of explanation, the examples in thedisclosure may be generally discussed with reference to an IC packageincluding an electronic logic circuit (e.g., a programmable IC) and anoptical communication circuit.

In some implementations, a logic circuit and a serializer circuit aremounted on a first interposer and an optical interface circuit ismounted on a second interposer. The first and second interposers aremounted on a package substrate having one or more wiring layers. Theserializer circuit is connected to the optical interface circuit via thefirst and second interposers and the wiring layers.

In some implementations, the optical interface circuit may include bothan optical transmitter and a driver configured to provide electronicdata signals to the optical transmitter for transmission. Some opticalcommunication circuits include a driver connected to an opticaltransmitter, and both the driver and optical transmitter are mounteddirectly on a package substrate and connected via wiring layers on thesubstrate. However, high speed communication though the wiring layerscan dissipate a significant amount of power. By including the opticaltransmitter and the driver on the same interposer, the transmission linedistance between the optical transmitter and the driver is reduced andpower efficiency improved. In some implementations, signal lines betweenthe optical transmitter and the driver have lengths less than or equalto ⅛ the wavelength of a highest frequency of the data signals.

In some implementations, the serializer circuit may instead be mountedon the second interposer with the optical interface circuit. Byintegrating the serializer circuit with the optical communicationcircuit on the same interposer, the data rate of the serializer circuitis not limited by the transmission through wiring layers of thesubstrate (e.g., 100 ohm differential signal lines). The logic circuitis connected to the serializer circuit by several parallel channels viathe first interposer, the wiring layers on the substrate, and the secondwiring layers. The serializer circuit is connected to the driver viawiring on the second interposer.

In some implementations, for example, the serializer circuit and thedriver are connected by signal lines having lengths less than or equalto ⅛ the wavelength of a highest frequency used by the serializercircuit to provide the serial data to the driver circuit. Use of signallines having lengths less than or equal to ⅛ the wavelength allowssignal lines to be implemented with higher impedances than the 100 ohmdifferential signal lines included in the wiring layers on thesubstrate.

Although the examples and implementations are primarily discussed withreference to a serializer circuit that provides serialized data fortransmission by an optical transmitter, the implementations are not solimited. For instance, the disclosed examples and implementations may beadapted for an optical receiver by replacing the serializer with adeserializer and replacing the optical transmitter with an opticalreceiver. Furthermore, the disclosed examples and implementations may beadapted for a transceiver configured to transmit and receive opticaldata. In such implementations, the serializer is replaced by aserializer/deserializer circuit (Serdes). For ease of explanation, theexamples and implementations are primarily discussed with reference to aserializer that provides serialized data for transmission by an opticaltransmitter. In some implementations, one of the two interposers may beomitted. For example, the first interposer may be omitted and the logiccircuit may be mounted directly on the substrate. Conversely, the secondinterposer may be omitted and the optical interface circuit may bemounted directly on the substrate.

Turning now to the figures, FIG. 1 shows a first IC package configuredin accordance with one or more implementations. In this example, a logiccircuit 120 and a serializer circuit 122 are mounted on a firstinterposer 110. An optical interface circuit, including a driver circuit124 and an optical transmitter 126, is mounted on a second interposer112. The interposers include contacts 114 (e.g., microbumps) forconnecting the circuits mounted thereon with external circuits. Thefirst and second interposers are mounted on a substrate 102 of thepackage. The logic circuit and serializer circuit may be communicativelycoupled to one with another by wiring layers (not shown) in theinterposer 110 and also communicatively coupled to the substrate by wayof through-silicon vias (not shown) and wiring layers in the interposerand the contacts 114. Driver circuit 124 and optical transmitter 126 aresimilarly communicatively coupled to one with another by wiring layers(not shown) in the interposer 112. The substrate includes one or morewiring layers (not shown) for interconnecting the interposers andexternal package terminals 104 (e.g., solderballs). In this example, thecircuits are encapsulated by a package cover 130. Alternatively oradditionally, a molding compound may be disposed over the circuits. Inthis example, the optical transmitter extends through the package cover130 to provide an optical data on the side of the package cover.

FIG. 2 shows a second IC package configured in accordance with one ormore implementations. The IC package in this example includes componentsthat are similar to those of the IC package shown in FIG. 1, asindicated by the reference numbers. In this example, the logic circuit120 and serializer circuit 122 are not mounted on an interposer (e.g.,110). Rather, the logic circuit 120 and serializer circuit 122 aredirectly mounted on the substrate 102.

FIG. 3 shows a third IC package configured in accordance with one ormore implementations. The IC package in this example includes componentsthat are similar to those of the IC package shown in FIG. 1, asindicated by the reference numbers. In this example, the opticalcommunication circuit (i.e., the driver circuit 124 and the opticaltransmitter 126) is not mounted on an interposer. Rather, the opticalcommunication circuit is directly mounted on the substrate 102.

FIG. 4 shows a fourth IC package configured in accordance with one ormore implementations. In this example, a logic circuit is mounted on afirst interposer 410. An optical interface circuit, including aserializer circuit 422, a driver circuit 424, and an optical transmitter426, is mounted on a second interposer 412. The interposers includecontacts 414 (e.g., microbumps) for connecting the circuits mountedthereon with external circuits. The first and second interposers aremounted on a substrate 402 of the package. The substrate includes one ormore wiring layers (not shown) for interconnecting the interposers andexternal package terminals 404 (e.g., solderballs). In this example, thecircuits are encapsulated by a package cover 430.

FIG. 5 shows a fifth IC package configured in accordance with one ormore implementations. The IC package in this example includes componentsthat are similar to those of the IC package shown in FIG. 4, asindicated by the reference numbers. In this example, the logic circuit420 is not mounted on an interposer (e.g., 410). Rather, the logiccircuit 420 is directly mounted on the substrate 402.

FIG. 6 shows a sixth IC package configured in accordance with one ormore implementations. The IC package in this example includes componentsthat are similar to those of the IC package shown in FIG. 4, asindicated by the reference numbers. In this example, the opticalcommunication circuit (i.e., the serializer circuit 422, the drivercircuit 424, and the optical transmitter 426) is not mounted on aninterposer. Rather, the optical communication circuit is directlymounted on the substrate 402.

Apparatuses and methods for serializing optical data signals are alsodisclosed. In some implementations, an optical serialization circuitincludes a plurality of optical modulators. Each of the opticalmodulators is configured to receive a respective bit of a parallelmulti-bit data bus in a first bit period. Each optical modulator isfurther configured to output a respective optical pulse representing thevalue of the received bit and having a duration less than the first bitperiod. For an N-bit data bus, the duration may be, for example, thefirst bit period divided by N.

The optical serialization circuit also includes a set of optical delaylines. Each delay line is configured to delay optical pulses produced byone of the optical modulators by a respective length of time unique tothe optical modulator to produce a respective optical output signal. Insome implementations, the respective lengths of time are multiples ofthe duration of the optical pulses. An optical combiner is configured tocombine the optical output signals to produce a single optical outputbeam.

In some implementations, the single output beam is transmitted throughan optical fiber to an optical deserializer. The optical deserializer isconfigured to receive the single optical beam and separate the opticaloutput signals from the single optical beam. The optical deserializer isfurther configured to provide the optical output signals as respectivebits of a parallel data bus.

Turning again to the figures, FIG. 7 shows an optical serializer, inaccordance with one or more implementations. The optical serializercomprises a plurality of optical modulators 720, 722, 724, and 726,which may be controllable lasers or optical multiplexors, such asMach-Zehnder modulators. Each of the optical modulators is configured toreceive a respective bit of an electrical N-bit data bus 702 in each bitperiod 710. Each optical modulator is configured to output a respectiveoptical pulse 750 representing the value of the received bit. Exampleoutput pulses 750 produced by the optical modulators 720, 722, 724, and726 are shown by waveforms 712. In this example, each of the outputpulses has a duration equal to 1/N of the bit period 710.

The optical serializer also includes a set of optical delay lines 730,732, 734, and 736. Each optical delay line is configured to delay theoptical pulses output by each optical modulator by a respective lengthof time unique to the optical modulator to produce a respective opticaloutput signal 752. In this example, each of the optical delay linesdelays optical pules by a respective multiple of the duration of theoutput pulses (i.e., bit period/N). Example output signals 752 are shownby waveforms 714. As illustrated by waveforms 714, each output pulsecoincides with a respective time period. An optical combiner 740 isconfigured to combine the output signals 752 to produce a combinedoutput signal 754.

Apparatuses and methods are disclosed for providing a configurableoptical channel with user configurable parameters. In someimplementations, an optical transmitter circuit includes a set ofoptical communication circuits, each configured to communicate opticaldata according to a different configuration of a parameter (e.g.,modulation, data rate, frequency, polarization, and/or phase). Theoptical transmitter circuit includes a selection circuit that isconfigured to select one of the set of optical communication circuitsfor operation in response to a first control signal. By selectingdifferent ones of the set of optical communication circuits at differenttimes, the operation of the optical transmitter circuit can be adjustedby way of the different configurations of the parameter.

In some implementations, the set of optical communication circuits mayinclude a plurality of lasers exhibiting respective characteristics. Forexample, in some implementations, each laser produces light of arespective frequency. By selecting different ones of the lasers foroperation, frequency of an optical data signal produced by the opticaltransmitter may be adjusted.

As another example, the set of optical communication circuits includes aplurality of optical delay lines, each configured to delay an opticaldata signal produced by the optical transmitter circuit by a differentamount of time. By selecting different ones of the optical delay lines,a phase of an optical data signal output by the optical transmitter maybe adjusted.

Turning again to the figures, FIG. 8 shows a configurable opticaltransmitter, in accordance with one or more implementations. In thisexample, the transmitter includes an electronic driver 804 configured toprovide an electronic signal to a plurality of lasers 810, 812, and 814for transmission. Each of the lasers is configured to output arespective optical data signal, encoding the electronic signal. Each ofthe lasers exhibits a unique configuration of an optical parameter. Forinstance, the lasers may exhibit respective frequencies, temperatureranges, and/or light intensities. The transmitter includes a selectioncircuit configured to select one of the lasers for operations. In thisexample, the selection circuit includes an optical multiplexor 818configured to output an optical data signal from one of the lasers thatis selected by control circuit 802. The optical multiplexor 818 blocksoptical data signals from other ones of the lasers. In some otherimplementations, the selection circuit may include a circuit configuredto enable a selected one of the lasers and disable non-selected ones ofthe lasers. An optical combiner may be used in lieu of an opticalmultiplexor 818 to merge optical data signals from the selected ones ofthe lasers.

In some implementations, the control circuit 802 may also adjust theconfiguration of various parameters of the electronic driver circuit804. For example, the electronic driver circuit 804 may be configured toadjust transmission rate and/or modulation algorithm used to encode datavalues (e.g., amplitude/frequency modulation) in response to controlsignals from the control circuit 802.

In this example, the optical transmitter also includes a set of opticalcomponents 822, 824, and 826 that may be selected to configure one ormore optical parameters of an optical data signal. The set of opticalcomponents 822, 824, and 826 may include, but are not limited to,optical delays, polarization filters, and/or spectrum filters.

In this example, an optical demultiplexor 820 is configured to providean optical data signal to one of the optical components 822, 824, and826, which is selected by the control circuit 802. An opticalmultiplexor 828 is configured to output an optical data signal from theone of the optical components 822, 824, and 826, which is selected bythe control circuit 802. The optical multiplexor transmits the selectedoptical data signal over an optical fiber 832.

In this example, the transmitter includes two respective sets ofcircuits that may be selected for operation (e.g., the set of lasers810, 812, and 814; and the set of optical components 822, 824, 826). Insome implementations, a transmitter may only include one set of circuitsthat may be selected for operation (e.g., either the set of lasers orthe set of components). Conversely, in some implementations, atransmitter may include three or more respective sets of circuits thatmay be selected for operation. In some implementations, opticalmodulators, such as Mach-Zehnder modulators, may be controlled in placeof the lasers, wherein the optical modulators control the intensity oflaser light sent to optical multiplexor 818.

The various implementations may be applicable to various applicationsusing optical data communication. As one example, a programmable IC mayinclude an input/output block configured to communicate data over anoptical fiber. FIG. 9 shows an example programmable IC that may beconfigured for optical communication in accordance with one or moreimplementations. This example shows a type of programmable IC known as aField-programmable-gate-array (FPGA). FPGAs can include severaldifferent types of programmable logic blocks in the array. For example,FIG. 9 illustrates an FPGA architecture (900) that includes a largenumber of different programmable tiles including multi-gigabittransceivers (MGTs) 901, configurable logic blocks (CLBs) 902, randomaccess memory blocks (BRAMs) 903, input/output blocks (IOBs) 904,configuration and clocking logic (CONFIG/CLOCKS) 905, digital signalprocessing blocks (DSPs) 906, specialized input/output blocks (I/O) 907,for example, clock ports, and other programmable logic 908 such asdigital clock managers, analog-to-digital converters, system monitoringlogic, and so forth. Some FPGAs also include dedicated processor blocks(PROC) 910 and internal and external reconfiguration ports (not shown).In some implementations, at least one of the IOBs 904 is configured tocommunicate optical data in accordance with one or more of the abovedescribed implementations.

In some FPGAs, each programmable tile includes a programmableinterconnect element (INT) 911 having standardized connections to andfrom a corresponding interconnect element in each adjacent tile.Therefore, the programmable interconnect elements taken togetherimplement the programmable interconnect structure for the illustratedFPGA. The programmable interconnect element INT 911 also includes theconnections to and from the programmable logic element within the sametile, as shown by the examples included at the top of FIG. 9.

For example, a CLB 902 can include a configurable logic element CLE 912that can be programmed to implement user logic, plus a singleprogrammable interconnect element INT 911. A BRAM 903 can include a BRAMlogic element (BRL) 913 in addition to one or more programmableinterconnect elements. Typically, the number of interconnect elementsincluded in a tile depends on the height of the tile. In the picturedexample, a BRAM tile has the same height as five CLBs, but other numbers(e.g., four) can also be used. A DSP tile 906 can include a DSP logicelement (DSPL) 914 in addition to an appropriate number of programmableinterconnect elements. An 10B 904 can include, for example, twoinstances of an input/output logic element (IOL) 915 in addition to oneinstance of the programmable interconnect element INT 911. As will beclear to those of skill in the art, the actual I/O bond pads connected,for example, to the I/O logic element 915, are manufactured using metallayered above the various illustrated logic blocks, and typically arenot confined to the area of the input/output logic element 915.

In the pictured example, a columnar area near the center of the die(shown shaded in FIG. 9) is used for configuration, clock, and othercontrol logic. Horizontal areas 909 extending from this column are usedto distribute the clocks and configuration signals across the breadth ofthe FPGA.

Some FPGAs utilizing the architecture illustrated in FIG. 9 includeadditional logic blocks that disrupt the regular columnar structuremaking up a large part of the FPGA. The additional logic blocks can beprogrammable blocks and/or dedicated logic. For example, the processorblock PROC 910 shown in FIG. 9 spans several columns of CLBs and BRAMs.

Note that FIG. 9 is intended to illustrate only an exemplary FPGAarchitecture. The numbers of logic blocks in a column, the relativewidths of the columns, the number and order of columns, the types oflogic blocks included in the columns, the relative sizes of the logicblocks, and the interconnect/logic implementations included at the topof FIG. 9 are purely exemplary. For example, in an actual FPGA, morethan one adjacent column of CLBs is typically included wherever the CLBsappear, to facilitate the efficient implementation of user logic.

The methods, circuits, and systems are thought to be applicable to avariety of systems and applications which utilize optical communication.Other aspects and features will be apparent to those skilled in the artfrom consideration of the specification. Though aspects and features mayin some cases be described in individual figures, it will be appreciatedthat features from one figure can be combined with features of anotherfigure even though the combination is not explicitly shown or explicitlydescribed as a combination. The methods, circuits, and systems may beimplemented as one or more processors configured to execute software, asan application specific integrated circuit (ASIC), or as a logic on aprogrammable logic device. It is intended that the specification anddrawings be considered as examples only, with a true scope of theinvention being indicated by the following claims.

What is claimed is:
 1. An apparatus comprising, a package substrate; afirst interposer mounted on the package substrate; a logic circuit; anoptical interface circuit connected to the logic circuit via the firstinterposer; and wherein: one of the optical interface circuit or thelogic circuit is mounted on the first interposer; and the opticalinterface circuit includes: a driver circuit configured to receiveelectronic data signals from the logic circuit; and an opticaltransmitter circuit coupled to the driver circuit and configured tooutput optical data signals encoding the electronic data signals.
 2. Theapparatus of claim 1, further comprising: a second interposer mounted onthe package substrate; the other one of the optical interface circuit orthe logic circuit is mounted on the second interposer; and the opticalinterface circuit is connected to the logic circuit via the firstinterposer and the second interposer.
 3. The apparatus of claim 2,wherein: the optical interface circuit further includes a serializercircuit configured to receive parallel data from the logic circuit,serialize the parallel data, and provide the serialized data to thedriver circuit as the electronic data signals.
 4. The apparatus of claim3, wherein the serializer circuit and the driver circuit are connectedby signal lines having lengths less than or equal to ⅛ the wavelength ofa highest frequency used by the serializer circuit to provide theserialized data to the driver circuit.
 5. The apparatus of claim 2,further comprising: a serializer circuit mounted on the first interposerand configured to receive parallel data from the logic circuit,serialize the parallel data, and provide the serialized data to thedriver circuit, via the first and second interposers.
 6. The apparatusof claim 1, wherein the optical transmitter circuit includes: a firstset of optical communication circuits configured and arranged tocommunicate optical data with a different setting of a first parameter;and a selection circuit configured and arranged to select one of thefirst set of optical communication circuits for operation in response toa first control signal.
 7. The apparatus of claim 6, wherein the firstset of optical communication circuits includes a plurality of lasers. 8.The apparatus of claim 6, wherein the first set of optical communicationcircuits includes a plurality of optical modulators.
 9. The apparatus ofclaim 6, further comprising: a second set of optical communicationcircuits configured and arranged to modulate an optical data signaloutput by the first set of optical communication circuits, each of thesecond set of optical communication circuits configured to modulate asecond parameter of the optical data signal by a respective amount; andwherein the selection circuit is further configured and arranged toselect one of the second set of optical communication circuits foroperation in response to a second control signal.
 10. A method,comprising mounting a logic circuit on a first interposer; forming anoptical interface circuit on a second interposer by mounting an opticaltransmitter circuit on the second interposer, mounting a driver circuiton the second interposer, and connecting the driver circuit via wiringon the second interposer; mounting the first interposer on a substrate;mounting the second interposer on the substrate; and connecting thelogic circuit and the optical interface circuit via the first interposerand the second interposer.
 11. The method of claim 10, wherein theforming of the optical interface circuit on a second interposer furtherincludes mounting a serializer circuit on the second interposer, theserializer circuit being configured to receive parallel data from thelogic circuit, serialize the parallel data, and provide the serializeddata to the driver circuit.
 12. The method of claim 11, furthercomprising: connecting the serializer circuit and the driver circuitusing signal lines having lengths less than or equal to ⅛ the wavelengthof a highest frequency used by the serializer circuit to provide theserialized data to the driver circuit.
 13. The method of claim 10,further comprising: mounting a serializer circuit on the firstinterposer, wherein the serializer circuit is configured to receiveparallel data from the logic circuit, serialize the parallel data, andprovide the serialized data to the driver circuit, via the first andsecond interposers.
 14. An apparatus, comprising: an optical serializer,including: a plurality of optical modulators, each configured andarranged to receive a respective bit of a parallel N-bit data bus, therespective bit having a first bit period, and output a respectiveoptical pulse representing a value of the respective bit and having aduration less than the first bit period, wherein N is greater than orequal to 2; for each of the plurality of optical modulators, arespective optical delay line configured and arranged to delay therespective optical pulses of the optical modulator by a respectivelength of time unique to the optical modulator to produce a respectiveoptical output signal; and an optical combiner configured and arrangedto combine the respective optical output signals produced by the opticaldelay line into a single optical beam.
 15. The apparatus of claim 14,wherein the respective optical pulse output to each of the plurality ofoptical modulators has a first duration equal to the first bit perioddivided by N.
 16. The apparatus of claim 15, wherein the optical delayline is further configured and arranged to delay output of each of theplurality of optical modulators by an amount of time equal to arespective multiple of the first duration.
 17. The apparatus of claim16, wherein for the output of one of the plurality of opticalmodulators, the respective multiple is equal to zero.
 18. The apparatusof claim 14, further comprising: an optical fiber having a first endconnected to an output of the optical combiner; an optical deserializerconnected to a second end of the optical fiber and configured andarranged to: receive the single optical beam via the optical fiber;separate the optical output signals from the single optical beam; andprovide the optical output signals as respective bits of a parallelN-bit data bus.
 19. A method, comprising: for each bit of parallel N-bitdata bus transmitted in a first bit period, using a respective opticalmodulator to provide a respective optical pulse having a duration lessthan the first bit period, wherein N is greater than or equal to 2;delaying each of the respective optical pulses by a respective length oftime unique to the optical modulator to produce a respective opticaloutput signal; and combining the respective optical output signals intoa single optical beam.
 20. The method of claim 19, wherein: therespective optical pulse has a first duration equal to the first bitperiod divided by N; and the delaying of each of the respective opticalpulses includes delaying each of the respective optical pulses by anamount of time equal to a respective multiple of the first duration.